Ultrasonic inspection methods, such as Scanning Acoustic Microscopy (SAM), provide valuable tools for the nondestructive inspection of microelectronic components and materials. By analyzing the ultrasonic response from a sample, the different interfaces and features of the sample can be verified. Also, flaws such as delaminations, cracks, voids, die tilt, underfill density variations and solder bump distortions can be detected. Ultrasonic analysis is usually performed by looking for changes in the amplitude or time-of-flight (TOF) of reflections in a high-frequency waveform signal (A-scan) of the sample at various locations (x, y) in the plane of the sample. By acquiring several A-scans along a line in this plane, a vertical cross-section image can be obtained. Also, by performing a raster-scan over the sample and only recording the amplitude or TOF from a certain depth within the sample at each location (x, y), a horizontal cross-section (C-scan) can be obtained. These images are easier to interpret than the set of A-scans, and are a common method for displaying ultrasonic data. More recently, full 3-dimensional renderings of a sample have become possible by recording the full A-scan at each location (x, y). These data sets allow for simulated scanning for efficient analysis, frequency-domain filtering to enhance of remove desired features, and F-scan imaging to bring out information that may be hidden in the time-domain signal.
All of these inspection methods currently rely upon the use of water to provide acoustic coupling between the ultrasonic transducer and the sample. The coupling may be provided by a flow of water from a dispenser or by immersion of the transducer and sample in a water bath. Typically, in the ultrasonic scanning of microelectronic parts, the water used for acoustic coupling is at ambient room temperature. However, the extent and shape of a flaw in a microelectronic sample may change when the sample is in use because the sample is at an elevated temperature.
The resolution of an ultrasonic scan is largely determined by the wavelength (frequency) of the ultrasound and the focusing ability of the ultrasonic transducer. However, the attenuation of ultrasound in the coupling medium between the transducer and the object being scanned increases rapidly with increasing frequency. Separations between the transducer and the object being scanned of much less than 0.5 mm are currently impractical in scanning acoustic microscopes. This separation sets an effective upper limit on the frequency of the ultrasound and consequently sets a limit on the spatial resolution that may be achieved in the ultrasonic scan.